How wood frogs freeze in winter and wake up in spring

In midwinter, a wood frog has no heartbeat, no breathing and no blood flow, and it is not dead. The Animal Diversity Web (ADW), a University of Michigan database, reports that all three stop while ice forms under the frog’s skin and among its muscles. Come spring the ice melts, the heart restarts, and the frog gets back to the serious business of finding a pond.

This is the animal that makes freezer manufacturers look underqualified. Many sources call it Rana sylvatica. We use Lithobates sylvaticus because that is the name on the IUCN record GBIF distributes, and the Animal Diversity Web and the Carnegie Museum of Natural History list both names for one and the same frog.

This article walks through what freezes, what protects the frog’s cells, how far the trick stretches in Alaska, and where the numbers disagree. Where sources conflict, we say so instead of picking a favorite.

Meet the wood frog

According to the Animal Diversity Web, the wood frog ranges from northern Georgia through the northeastern United States, across Canada and into Alaska. It breeds in fish-free seasonal or semi-permanent pools and spends winter in sheltered ground cover. The same page says it lives farther north than any other North American reptile or amphibian, and Nature News describes it as reaching the Arctic Circle.

Its breeding style is as dramatic as its winters. The page says wood frogs emerge in late winter or early spring, often calling before the ice leaves the ponds. Another early starter is the spring peeper, whose choruses begin in some places before the thaw is finished, but it is a smaller frog.

What actually freezes

Here the sources disagree, and we cannot fully reconcile them. The Animal Diversity Web says up to 35 to 45 percent of the body can freeze. Nature News says the frogs survive freezing of more than half of their body water. The Carnegie Museum, in a short blog post, says they freeze two-thirds of their bodies. These are loose summaries, and none gives enough detail to compare them.

What the pages agree on is the pattern. The ADW places the ice under the skin and among the skeletal muscles. A 1995 review by Jack Layne and Richard Lee, which covers five freeze-tolerant North American frogs (the abstract we read does not name which five), adds that organs lose half or more of their water in the first 24 hours of freezing. That concentrates the protective chemical, called a cryoprotectant, and reduces ice damage. The same abstract says the heat released as body water freezes keeps the circulation working for hours.

Then comes the part no one forgets. Breathing and heartbeat stop. The frogs recover within a few hours of thawing, according to the review.

The wood frog is not the only freeze-tolerant frog on this site. A conference abstract from Central Washington University and Indiana University Kokomo reports that spring peepers also survive freezing, with survival depending largely on glucose that the liver releases from stored sugar. In that study, nearly all peepers survived freezing at minus 1.5 degrees Celsius, but few survived minus 2.5. The experiments differ from the wood frog work below, so these numbers are not a ranking.

Sugar and urea: the antifreeze

Glucose is the classic protector. The Layne and Lee abstract says freezing triggers the liver to release glucose or glycerol, up to about 100 times normal, and that this shields tissues from freezing injury. The same database adds specialized proteins that help prevent ice from forming inside cells.

Urea is the newer finding. In 2005, Jon Costanzo and Richard Lee of Miami University reported that urea levels rise sharply toward winter and peak at about 50 times summer levels during hibernation, as Nature News describes it. Urea at hibernation concentrations protected cells in freeze-thaw tests about as well as glucose did.

Two cautions come with that result. The protection was shown in isolated blood and tissues, not whole frogs, and the mechanism is unknown. Nature News also notes that urea is normally a waste product, and that the frogs’ levels seem too low to cause harm, without explaining why.

Alaska: pushing the limits

In 2014, Don Larson and Brian Barnes of the University of Alaska Fairbanks reported on wood frogs overwintering in leaf litter in Interior Alaska, where, per the university’s release, temperatures stay below freezing for more than six months. The release says minimums reach about minus 4 degrees Fahrenheit (minus 20 degrees Celsius).

The release reports that the frogs survived as long as 218 days frozen, and that lab frogs survived minimums of minus 18 degrees Celsius (about 0 degrees Fahrenheit). Earlier studies of frogs from the eastern United States and Canada found survival of only a few weeks, down to about minus 7.2 degrees Celsius (19 degrees Fahrenheit). Frogs that wintered outdoors held 13 times more glucose in muscle, 10 times more in heart and 3.3 times more in liver than lab-frozen frogs.

Why? Larson’s proposal is that repeated overnight freezing and daytime thawing in early autumn stimulates glucose release. Barnes said it is still open whether Alaska frogs differ because of temperature patterns or because of genetic differences, and that the question awaits further study. We read the release and a summary of it, not the paper, so we have not checked these figures there.

What this looks like in practice

Here is a winter, step by step, as the sources describe it.

For example, an Alaska frog starts the season with a glucose boost from cold autumn nights, loads up on urea, freezes in leaf litter, and still shows up for the breeding party. For a different answer to the same winter problem, see the painted turtle, which hibernates underwater beneath ice.

Common mistakes and myths about frozen frogs

Myth: a frozen frog is a dead frog. Breathing and heartbeat stop, but the sources describe recovery within hours of thawing. The frog is paused, not finished.

Mix-up: the whole frog turns to a block of ice. The percentages differ by source, but none says all water freezes. The Animal Diversity Web says glucose and specialized proteins prevent ice from forming inside cells and prevent dehydration.

Mistake: every wood frog tolerates the same cold. The Alaska work found much longer survival than earlier studies of southern frogs, and Barnes says the reason is still unresolved.

Myth: this will soon let us freeze humans. The Alaska release mentions organ transplantation only as a speculative idea for the future.

Mix-up: two names, two animals. Rana sylvatica and Lithobates sylvaticus are the same frog.

Mix-up: frogs that share a genus name are look-alikes. The American bullfrog is also listed under Lithobates by the Animal Diversity Web, yet that page calls it the largest true frog in North America, while the wood frog tops out at 7.6 centimeters.

Conservation status

The wood frog is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category.

Least Concern is not the same as no problems. The ADW says it is a species of special concern in some areas, and names habitat loss to agriculture and development, drained breeding ponds, logging, road deaths during migration, and acid rain or toxic runoff that harms eggs and larvae. We have no confirmed population trend, because none of our sources states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the assessment, so we do not state one.